The present invention relates to a method for processing a lithographic imaging material with a liquid. More preferably, the invention relates to a method of wet processing a lithographic printing plate material that can be performed while the material is contained in a printing press.
Lithographic printing presses use a so-called printing master such as a printing plate that is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called xe2x80x98computer-to-platexe2x80x99 (CTP) method has gained a lot of interest. This method, also called xe2x80x98direct-to-platexe2x80x99, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. A special type of CTP involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of a plate-setter that is integrated in the press. This method may be called xe2x80x98computer-to-pressxe2x80x99 and printing presses with an integrated plate-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science and Technology""s 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364.
Especially thermal plates, which are sensitive to heat or infrared light, are widely used in computer-to-plate and computer-to-press methods because of their daylight stability. Such thermal materials preferably comprise a compound that converts absorbed light into heat. The heat, which is generated on image-wise exposure, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex, and after optional processing, a lithographic image is obtained. Thermal plate materials, which are often used in computer-to-press methods, are based on heat-induced ablation. A problem associated with ablative plates is the generation of debris that is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. As a result, such ablative plates require a processing step for removing the debris from the exposed material.
A non-ablative plate which can be developed with fountain and ink is described in EP-B 770 494. The latter patent specification discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby converting the exposed areas into an hydrophobic phase which define the printing areas of the printing master. The press run can be started immediately after exposure without any additional treatment because the layer is developed by interaction with the fountain and ink that are supplied to the cylinder during the press run. So the wet chemical processing of these materials is xe2x80x98hiddenxe2x80x99 to the user and accomplished during the first runs of the printing press. A problem associated with such non-ablative, hidden-processing materials is the low run length of the plate.
Other thermal plates, e.g. Agfa""s Thermostar(trademark) or Electra(trademark) from Kodak Polychrome Graphics, provide a longer run length but require wet processing with alkaline liquids which may damage or contaminate the electronics and optics of the integrated image-setter and other devices of a digital printing press. Also UV-sensitive plates would be very suitable for CTP because of their potential stability in visible or yellow light. In addition, most conventional UV-sensitive, so-called pre-sensitized (xe2x80x9cPSxe2x80x9d) plates, which are normally used for contact exposure through a film mask, enable a very high run length in addition to other benefits such as high speed, resistance to press chemicals, and better physical resistance, e.g. towards scratching, than typical thermal plates. However, such PS plates also require alkaline processing, which makes them less suitable for on-press processing as required in computer-to-press methods. Such plates are therefore usually developed in a dedicated processing apparatus and then mounted on the press. This requires quite some plate handling during mounting and registering of the plates and therefore involves a risk of scratches, fingerprints or other damage or contamination of the lithographic image.
So there is a need for a wet processing method which can be performed while the imaging material is mounted near to optical, electronic or other fragile components, such as in a plate-setter or digital printing press, without the risk of damaging or contaminating such devices. Such a processing method opens computer-to-press systems to plates that require wet processing but on the other hand are characterized by benefits such as long run length or daylight stability.
It is an object of the present invention to provide a method of wet processing which can be carried out on a lithographic imaging material, wherein the risk of damaging or contaminating nearby electronic and optical components is minimized. This object preferably is achieved by the characterizing features of the present invention. Advantageous embodiments and further developments of the solution will be apparent from the description of the invention provided herein. According to the method of the present invention, a processing liquid is supplied to a lithographic imaging material which comprises a support and a heat- or light-sensitive coating, which may consist of one or more layer(s) provided on the support. The material forms a latent image upon image-wise exposure, preferably without substantially ablating the coating, i.e. without removing an amount of substances from the coating during the exposure that would be sufficient to create a useful image before the wet development step. The latent image consists of areas in the coating from which substances can be removed by the processing liquid and other areas from which these substances are not removed in the development step. The processing liquid is evacuated by means of a suction device together with one or more substance(s) that have been removed from the coating. Substances are removed from the coating by the processing liquid either at the exposed or at the non-exposed areas. The term xe2x80x9cremovedxe2x80x9d or xe2x80x9cremovablexe2x80x9d indicates that substances are or can be removed from the coating by the supply of the processing liquid by dissolution of said substances in the liquid or by the formation of a suspension, dispersion or emulsion of said substances in the liquid.
The benefits of the methods of the present invention are mainly achieved by the use of a suction device that collects the processing liquid from the material together with substances from the developed coating. The suction device can simply be a porous material such as a sponge or a liquid-absorbing cloth. Such porous material can be supplied from a roll and, after contacting the imaging material, be wound up on another roll. The supply of processing liquid and the collection thereof by suction can also be carried out with an apparatus as shown in the figures, which are discussed in more detail below. Such an apparatus is preferred over the use of a porous material since it reduces the risk of linting of clothes and the risk of mechanical damage or contamination of the lithographic support.
More specifically, the present invention provides a method of wet processing which can be carried out on a lithographic printing plate precursor that is mounted in a plate-setter or a digital printing press, e.g. while the plate precursor is mounted on the plate cylinder of a printing press. Besides the reduced risk of damaging nearby electro-optical components, such an on-press processing method provides the additional benefits that the plate does not need to be handled after exposure and processing and that it is not necessary to apply a so-called desensitizing coating (gum layer) to protect the developed plate against fingerprints or other contamination. Since the processing liquid and any substances from the coating contained therein are removed by the suction device, there is no risk of contaminating the ink or the fountain that is used in the subsequent printing step.
A preferred lithographic imaging material is a lithographic printing plate precursor which can form a lithographic image consisting of two sets of areas which have opposite affinity towards ink or an ink-abhesive fluid (i.e. hydrophilic and hydrophobic areas for wet offset printing or oleophilic and oleophobic areas for driographic printing). Such a lithographic image can be obtained after a wet development step wherein a layer having a first affinity towards ink or an ink-abhesive fluid is removed from the material and thereby another layer, or the support, is revealed which has the opposite affinity towards ink or an ink-abhesive fluid. The methods of the present invention are suitable for removing ablation debris from the exposed printing plate precursor, wherein it may suffice to use water as a processing liquid. In a more preferred embodiment, the printing plate precursor is non-ablative, i.e. the material shows no substantial ablation of one or more layer(s) upon exposure. xe2x80x9cNo substantial ablationxe2x80x9d means that no useful lithographic image is formed after exposure by ablative removal of a layer. Instead, the preferred plate precursors used in the present invention form a useful lithographic image only after removal of exposed or non-exposed areas of the coating by means of a wet development step.
The step of image-wise exposure can involve exposure to electromagnetic radiation or to heat, since the method of the present invention is applicable to both light- and heat-sensitive materials. Preferred light-sensitive materials are UV-sensitive, especially UV-sensitive materials which can be handled in daylight. For similar reasons, heat-sensitive materials that can be exposed with a thermal head or infrared radiation are also very suitable. For the image-wise exposure, laser-scanning devices are preferred, especially UV and infrared light emitting lasers. Also UV lamps, preferably digitally modulated, e.g. by means of light valves or a digital mirror device, can be used.
The processing liquid can be applied to the exposed imaging material by dipping or immersion, but coating methods are especially preferred over dip tanks because of the lower consumption of processing liquid and the smaller probability of degradation of the liquid, e.g. by oxidation, uptake of carbon dioxide, etc. More specifically, the processing liquid can be jetted or sprayed, e.g. by means of an apparatus as shown in the figures, or applied by roller coating. A preferred processing liquid is an aqueous alkaline solution, having a pH in the range 7.5-14. The processing can be achieved by chemical as well as mechanical effects. Both effects are preferably combined, e.g. the mechanical impact of jetted processing liquid may facilitate the removal of the layers from the material. Other suitable mechanical treatments may involve rubbing, brushing, or ultrasonic treatment. Mechanical treatment is advantageous since it provides the possibility to reduce the chemical reactivity of the processing liquid, e.g. to reduce the pH or the temperature thereof.
According to still another method of the present invention, a film of a processing liquid is applied on the coating as described above, but without using the suction device and then, after a time period of preferably at least 10 seconds, more preferably between 20 and 90 seconds, during which the processing liquid attacks the coating, the processing liquid is removed by the suction device together with removable substances from the coating.
The development step may be followed by an optional rinse step, e.g. with water applied by a pressure-washer, and an optional drying step, e.g. by applying hot air or by using infrared lamps. The development step may also be preceded by a pre-treatment such as soaking the layer(s) of the imaging material with water or another liquid.